We have continued our work on the developmentally regulated transcription of Plasmodium ribosomal rRNA genes. We have defined the pattern of expression of the different units during parasite development in the mosquito. We have found that up-regulation of one unit relates to the differentiation of the sporozoite as a unit within the oocyst. We have also discovered a previously undescribed rDNA unit that is expressed only during early oocyst development in the mosquito. In separate studies, we have continued our work with attenuated lines of human and rodent malarias. Auxotrophic lines of parasites have been cloned and their developmental characteristics studied. Auxotrophic mutants of both species of parasite have been defined by their dependency on either exogenous pyrimidines or para-aminobenzoic acid . These should prove useful in the study of both the biochemistry of parasites and the development of immunity to parasites.